This invention relates to a method and apparatus for soldering. It is particularly concerned with the manufacture of circuit boards.
The manufacture of circuit boards is a well known industrial operation. Typically, electrical circuits are printed onto a board, and holes are drilled in the board at chosen locations to enable chosen circuit components to be inserted and to be soldered to electrical conductors on the board. The soldering operation is typically performed automatically or semi-automatically. Boards to be soldered are loaded in sequence onto a conveyor which advances them through an elongate chamber housing a soldering station. The soldering therefore takes place as the boards are advanced through the chamber.
There are two commercially important methods of soldering: reflow soldering and wave soldering. In fellow soldering, miniature electronic components are surface mounted on a printed circuit board to which a solder in a creamy or paste-like consistency has been applied by a method such as screen printing, stencilling or dispensing. The printed circuit board is then subjected to a sufficiently high temperature, generally 50.degree. C. greater than the melting point or liquidus of the solder alloy, to cause the alloy to liquery and to contact the components such that upon subsequent cooling of the printed circuit board, the components remain secured in place on the board by the solder. The heat can be supplied by, for example, infrared, vapour phase, heated conveyor belt or convective means. The solder conventionally comprises a soft powdered metal alloy dispersed in a liquid medium containing a flux, an organic solvent and a thickening agent selected to impart the desired consistency to the mixture.
Have soldering differs from reflow soldering in that the solder is not applied to the board in advance of the board's being carried through the soldering chamber. Instead, a vessel containing molten solder is located within the chamber and is arranged in conjunction with a pump to create a wave or waves of the solder. The circuit boards are each advanced through the soldering chamber in such spaced relationship to the wave or waves that appropriate amounts of solder are applied, as desired, to the components and that suitable solder joints are formed on cooling the boards. Typically, the boards are pre-treated with a flux at desired locations so that the solder is able to adhere to the surface at these locations.
In recent years, considerable research and development activity has been focused on optimizing the atmosphere in which the soldered joints are formed. In, for example, conventional wave soldering, the molten solder is oxidized when exposed to molecules of oxygen. The oxidized solder tends to form a surface oxide layer which is removed typically by a flux applied to the board before it is passed into the soldering chamber. Removal of the oxide enables the solder to wet the components to be soldered. Since the wave has a surface layer of molten solder which is continuously broken, a residue of oxide and solder, known as dross, collects within a vessel in which the solder is contained. Dross generation adds considerably to the cost of the wave soldering process owing to the lost value of the solder and the maintenance requirements it imposes. For example, it is necessary from time to time to remove dross from the vessel and to repair mechanical parts of the wave soldering apparatus damaged by its abrasive action. In addition, the dross itself can be a health hazard if the solder contains toxic components such as lead.
It has accordingly been proposed to blanket the entire surface of the wave with an atmosphere which is less reactive than air towards the solder. There is, however, a body of teaching in the art which requires the presence of oxygen in the soldering atmosphere. In particular, U.S. Pat. No. 4, 610,391 relates to an improvement in a process for wave soldering a workpiece in an atmosphere consisting essentially of air wherein (i) there is a first portion of the solder wave in which fluid motion can be observed, said first portion including an active dross forming area, and (11) there is a second portion of the solder wave which is the last portion of the solder wave with which the workpiece comes into contact. The improvement comprises replacing the atmosphere in contact with at least 50% of the surface of the active dross forming area with an inert gas, but preventing the atmosphere in contact with the surface of the second portion from becoming inert. Typically, two gas diffusers are employed. A first gas diffuser supplies an inert gas such as nitrogen to the front of the wave. A second gas diffuser applies a gas mixture typically containing from 18 to 50% by volume of oxygen (a balance typically being nitrogen) to the rear of the wave.
In EP-A-361 507, it is disclosed that in the presence of a soldering flux it is desirable to maintain an oxygen concentration in the range of from about 10,000 to 20,000 parts by volume per million in the vicinity of the solder wave. EP-A-0 330 867 discloses that this oxygen concentration can be as low as 10 parts by volume per million in a wave soldering process which employs no pre-treatment of the circuit boards or other components with flux.
EP-A-0 389 218 discloses that reflow soldering may be carried out in a low oxidizing atmosphere, preferably one containing no more than 1500 parts per volume per million of oxygen.
Notwithstanding the teaching in EP-A-0 330 867 A that wave soldering can be conducted without the pre-treatment of the printed circuit board with flux when the soldering atmosphere contains in the order of 10 parts by volume per million of oxygen, manufacturers still prefer to pre-treat the circuit boards with flux. The reasons are two-fold. First, difficulties arise in reliably and repeatedly being able to maintain in the soldering chamber an atmosphere having an oxygen concentration as low as 10 parts per million by volume even if, say, a source of substantially pure nitrogen (containing no more than 5 volumes per million of oxygen) is used to create the soldering atmosphere. Secondly, it is still found that some dross is formed even in low oxygen environments.
Although soldering flux is able to cater for the presence of oxygen in a soldering atmosphere, we nonetheless believe that there is a need to keep down the ingress of air into the soldering atmosphere. It is known to provide curtains at both the entrance to and exit from a soldering chamber in order to limit the ingress of air. Such curtains form a physical barrier to the ingress of air when nitrogen is supplied to the interior of the chamber. The curtains typically each comprise a number of fingers or filaments of rubber, thin metal sheet or glass fibres. The curtains typically totally obturate the entrance and exit to the chamber but are able to be displaced by advancing circuit boards to permit their entry to and exit from the chamber. We have surprisingly found that contact between such curtains, even though they are lightweight, and boards passing into the soldering chamber can displace some components sufficiently to impair their soldering and hence the quality of the finished circuit board. Although this problem can be overcome, by removing the curtains and increasing the flow of nitrogen used to form the soldering atmosphere, this practice leads to an undesirably high consumption of nitrogen if it is desired to maintain low oxygen concentrations within the soldering atmosphere. Me further believe that it is desirable to maintain a soldering atmosphere having a low oxygen concentration even if the boards are pre-treated with a soldering flux.
There is therefore a need in the art for an improved method and apparatus for soldering which facilitates the creation of soldering atmospheres containing only low levels of oxygen, but which overcome the above-described problem that arises with conventional curtains. The invention aims at meeting this need.